The present invention relates to apparatus for preventing light fogging of unexposed photographic film in its dispenser, particularly as encountered in archival microfiche film systems employing an intermediate filmstrip to carry a transferable image.
There has been developed by Klose and Ovshinsky (U.S. Pat. No. 4,123,157 issued Oct. 31, 1978) an archival hard copy microfiche recording system which reflects light off of a document placed on a document-receiving platform and projects an image of it in greatly reduced form onto an initially transparent intermediate dry-silver filmstrip carried by a film head unit. The filmstrip is dispensed from a cassette mounted on the film head, and is fed by film advancing means to an imaging region where a selected area of the filmstrip to be exposed can receive the projected light image. A pressure element presses the selected area of the filmstrip against a planar backing aperture defining a projection plane and an imaging area, and the film is then exposed to the light image. The film head is next moved along guides from the imaging station to a developing station, where a hot shoe pressing against the exposed area of the film causes development of the image to form an opaque image in the exposed areas thereof. The film head is then moved to an image transfer station, where the image on the dry-silver film is to be replicated on a positive initially opaque microfiche film card of the photo-developing type (See U.S. Pat. No. 4,137,078 issued June 30, 1979 to Izu and Ovshinsky). The microfiche card is held indexed on a movable carriage so that a predesignated frame of the card receives the image on the intermediate film. A pressure element presses the film card and the intermediate filmstrip together between the output prism of a flash lamp housing and a backing plate. Image transfer is accomplished by triggering the flash lamp to direct light through the intermediate film, immediately rendering transparent the previously opaque areas struck by the high intensity flash lamp light on the selected frame of the microfiche film card. To replicate a new image the film head is returned to the imaging station, where the cycle is repeated.
The microfiche film cards are of the threshold photo-developing type, and require a relatively high radiant flux for complete exposure compared to the dry silver intermediate film used. The transfer of the intermediate image to the microfiche film is done by contact printing, using a high intensity flash lamp. A fraction of this illumination becomes trapped in the intermediate film, which is quite clear in unexposed regions, and proceeds to propagate laterally internally along the plane of the film by total internal reflection (light-piping), thereby making its way into unexposed film in the payout region of the film dispenser. A significant length of intermediate film thus becomes light struck in the film dispenser. This necessitates a long film advance between successive frames, and is thus wasteful of film.
This phenomenon is not significant during formation of the initial image because the requisite illumination level is much lower than for the transfer operation, owing to the relatively high speed of the intermediate film compared to the microfiche film. During the transfer operation the intermediate film is subjected to an illumination level several orders of magnitude higher than necessary for initial image formation, and the propagation of a small fraction of this by light-piping is sufficient to give rise to significant fogging effects. Additionally, the intermediate film is similarly fogged by prolonged exposure of the cassette to the ambient light of the system. It is an object of this invention to protect the microfiche record against the effects of fogging from both sources.
A conventional anti-fog approach such as the application of an overall light-absorbing layer (antihalation backing) on the film face opposite the photosensitve layer is inappropriate to dry-process films of the heat-developing kind. There is no obvious or practical way to remove such a layer to allow final image transfer. In conventional wet chemistry processing of photographic film the layer is dissolved away during the development process.
A pending application of Ovshinsky and Klose (Ser. No. 207,229, filed Nov. 17, 1980) discloses a variety of methods for producing transmission barriers in a dry-process intermediate filmstrip between the imaging area and the remainder of the unexposed film. These barriers take two basic forms. One is to produce reflection absorbing bands on the surface of the film, as by inking, or within the surface region of the film by local exposure and development of the photographic emulsion. Another version employs serration or other mechanical deformation of the film to destroy local film planarity and cause the trapped light to escape from the film or be reflected backwards.
Both approaches are successful, but require either additional special equipment in the microfiche system or special film manufacturing techniques not routinely employed by the photographic film industry. Accordingly, it is an object of this invention to provide adequate barriers in film by employing those techniques commonly used in the manufacture of photographic film.